ඊනියා යථාර්ථය
මේ නිව් සයන්ටිස්ට් සඟරාවේ දැනුමේ සීමා යටතේ පළ වූ ලිපි පහකින් හතරවැන්නයි. යථාර්ථයක් නැති කම යථාර්ථය දැයි කිසිවකු ප්රශ්න කිරීමට ඉඩ තියෙනවා.
How
can we understand quantum reality if it is impossible to measure?
If
we can’t measure something, we can’t know its true nature. This fundamental
limitation hampers our understanding of the quantum world – but it doesn't
preclude scientific thinking
PHYSICS 10 January 2023
Jamie Mills
Most
of us intuitively feel that reality ought to exist just fine on its own when we
aren’t looking. If a tree falls in a forest when no one is around to hear the
crash, the air still vibrates with sound waves, right? Yet it is a tricky
proposition to prove and one that gets more slippery when you consider things
that seem to exist, but that we will never be able to observe. Grappling with
the question of how to measure the immeasurable can, however, help us see what
reality is truly like.
There
are a few realms where the laws of nature themselves forbid us from treading.
Nothing can travel faster than the speed of light, which means we will never see
beyond the edge of the observable universe – the maximum distance that light
can have traversed to reach our telescopes since the beginning of the
universe. General relativity rules that nothing within a
black hole can escape, so that is another no-go zone (see “What is inside a black hole?”).
This
article is part of a special series on the limits of knowledge, in which we
explore:
How AI is shifting the limits
of knowledge imposed by complexity
Why maths, our best tool to
describe the universe, may be fallible
Why some aspects of physical
reality must be experienced to be known
Logic underpins knowledge – but
what if logic itself is flawed?
But
perhaps the most fundamental limit to what we can measure comes from the laws
of quantum physics. These tell us that if we measure some
property of a quantum particle today, it is impossible to know if we will get
the same result when measuring it with an identical set-up tomorrow. In this
sense, the laws of quantum mechanics aren’t like Isaac Newton’s classical laws
of motion, which give definite predictions (see “When things are outrageously complicated” ).
Instead, they can only predict how stuff behaves on average.
The
measurement problem
The
traditional interpretation of these facts is that particles exist in a cloud of
many possible states at once, described by a mathematical construct known as a
wave function. The idea is that the wave function only collapses into a single
state, to certainty, upon measurement. If so, before we look at it, reality is
a kind of fog of possibilities and our knowledge of it is blurry at best.
But
not everyone agrees with that. For Vlatko Vedral, a physicist at the University
of Oxford, it is a mistake to make a distinction between a particle that
adheres to the rules of quantum theory and the observer or measurement
apparatus that follows the laws of classical physics. He reckons that,
ultimately, everything is quantum and we should see reality as one gigantic,
universal wave function.
If
we accept that wave functions are the essence of reality, this casts quantum
physics in a new light. Everything has a wave function and all of them are
quantum entangled with each other, meaning a measurement of one affects the
others. So we can’t think about measuring isolated objects in the traditional
scientific sense because the measurement apparatus and the object being
measured always interact. In other words, reality as we see it is a product of both
the observer and the object under scrutiny, rather than some stand-alone real
view of that object, which seems beyond us. “You can only ever isolate systems
imperfectly,” says Noson Yanofsky at Brooklyn College in New York. “That leads
to a lack of knowledge.” Whether this is really a limitation, though, depends
on your point of view. “It’s only a limitation if you’re thinking in terms of
these old concepts,” says Vedral.
Relational
quantum mechanics
Many
physicists would agree that drawing a clear line between small quantum objects
and larger classical ones is problematic. But opinions differ on precisely how
to interpret the quantum realm. Carlo Rovelli at Aix-Marseille University in
France doesn’t think the wave function is a real object. He has been working on
an alternative idea known as the relational interpretation of quantum mechanics. In this
view, everything in existence exists only in relation to other things,
including you. A particle is there when you measure it, but doesn’t exist at
all times. “To ask what an electron’s momentum is can simply be a meaningless
question,” he says.
Read
more: Carlo Rovelli on the bizarre world of relational quantum mechanics
We
may never prove which interpretation is right, because our observations of the
quantum world seem to change it. But there may be ways to progress. Rovelli
argues that we can use the different interpretations to clarify existing
puzzles in physics. For instance, he suspects that his relational view could help align the two famously incompatible
pillars of modern physics: quantum theory and general relativity. If
it does, that is a good reason to favour it.
Then
there is the possibility – albeit a slim one – that one day we will find a way
of seeing into the quantum fog without collapsing the wave function. If the
laws of physics say something is impossible, that is only valid as long as the
laws themselves hold. A deeper version of quantum theory could come along and
make the impossible possible. But Vedral isn’t holding his breath that a
definite, predictable world will reappear. “It’s probably going to be even
weirder,” he says.
If you fall into a black hole, you aren’t coming back. That
goes not just for people, but any measuring device or unfortunate craft we
might send into the void. Past the black hole’s iconic event horizon, gravity
becomes so strong that objects are “spaghettified”. Still, there might be a
loophole by which something can come back out.
Decades ago, Stephen Hawking figured out that black holes slowly
evaporate by emitting radiation. If a black hole evaporates entirely, then the
information it had absorbed over the aeons would have been obliterated – but a
key law of nature is that information can’t be destroyed. This is the essence
of the black hole information paradox.
Physicists remain sharply divided on what to make of it. We have
photographed black holes, modelled them using exotic fluids here on Earth and
chipped away at the problem theoretically. Still, the goal is that by studying
what happens right at the edge of the event horizon, we can inch closer to
solving it. In the process, many hope we might get a glimpse of a new theory of
quantum gravity, one that surpasses both quantum mechanics and general
relativity.
